Digital content provision

ABSTRACT

A collaborative recommender system which is based on an application of Formal Concept Analysis is disclosed. A problem with collaborative recommender systems based on Formal Concept Analysis is that the size of the concept lattices generated can be very large. Large lattices both take a great deal of time to generate, and tend to include extraneous concepts. In the collaborative recommender systems disclosed herein the size of the concept lattices used are only of a scale required to capture the tastes of an individual user. This reduces the computational resource required by the recommender system markedly. Further improvements are obtained in some embodiments by selecting only relevant concepts from the concept lattice before building a user profile from the selected concepts. Yet further improvements are obtained by the introduction of a relevance measure which depends upon the difference between the size of the extent of the concept whose relevance is being measured and the size of the largest lower neighbour of the concept whose relevance is being measured. This relevance measure can be computed more easily than prior relevance measures.

This application is the U.S. national phase of International ApplicationNo. PCT/EP2016/064598 filed Jun. 23, 2016, which designated the U.S. andclaims priority to EP Patent Application No. 15275164.0 filed Jun. 26,2015, the entire contents of each of which are hereby incorporated byreference.

BACKGROUND AND SUMMARY

The present invention relates to selecting digital content for provisionvia a communication network to a user. It has particular utility invideo on demand systems.

Recommendation systems are often provided as part of a user's interfacewith video on demand systems. Some known recommendation systems arecollaborative recommender systems, which identify users with similartastes to the target user, and then recommend digital content whichthose other similar users have rated highly.

An important component of such algorithms is the identification ofsimilar users. Large video on demand systems can serve tens of millionsof users, and have access to billions of individual content ratings.Identifying similar users and predicting ratings for a given set oftarget items for a target user then becomes a problem requiring a greatdeal of computational resource.

Classification of users into groups can reduce the complexity of theproblem—by identifying a group of users to which the current userbelongs, films which the user has not yet seen but which are popularwith members of that group can be identified and recommended to a user.An example of this approach is seen in US patent application US2011/0061069 which generates clusters of users based on clustering theattributes of films or televisions programs which they have watched(their viewing history being collected by the set-top box in theirhome).

One method of categorization known as Formal Concept Analysis, aims atgrouping objects described by common attributes. Formal Concept Analysis(FCA) constructs a concept hierarchy of a group of objects, based onsubsets of objects having shared attribute sets. The core FCAdefinitions are outlined below.

Definition 1. (Formal Context) A formal context is a set of the binaryrelationships between the set of objects and the attributes theyindividually possess.

Definition 2. (Formal Concept) A formal concept is a pair (A, B). Theset of objects A is referred as the extent of the formal concept whereasthe set of attributes B is referred to as the intent.

Definition 3. (Subconcept and superconcept) A partial order relation, ≤,for context K may exist between a pair of formal concepts and isdescribed by subset relation between their extents or a supersetrelation between their intents.

Essentially (A₁,B₁)≤(A₂,B₂) if all objects of A₁ are found within theset A₂ or if all attributes of the set B₂ are within the set B₁. (A₁,B₁)would be a subconcept of (A₂,B₂) whereas (A₂,B₂) is described as asuperconcept of (A₁,B₁).

Definition 4. (Upper and lower neighbour) If (C,D)≤(A,B) and there existno (E, F) such that (C,D)≤(E, F)≤(A,B) then (C,D) is a lower neighbourof (A,B) and (A,B) is an upper neighbour of (C,D).

Definition 5. (Concept lattice) Alongside a unique greatest commonsubconcept (infimum) of the set of all formal concepts in K, and aunique least common superconcept (supremum) of that same set, thepartial order relation between all formal concepts of formal context Kcan be displayed as line diagram or lattice. L(K)=(K, ≤) is the conceptlattice of the formal context (G,M, I) where G and M are sets and I is asubset of the cross product of G and M.

The binary relations of a formal context K:=(G, M, I) may be representedin the form of a table where the rows represent the objects, the columnsthe attributes, and an X in the cell representing the existence of abinary relation between the corresponding attribute and object.

Concept lattices have been proposed as a suitable way of organisinginformation about users and the films they have watched or rated. Thepaper ‘Collaborative Recommending using Formal Concept Analysis’ byPatrick du Boucher-Ryan, and Derek Bridge presented in Knowledge-BasedSystems, Volume 19, Issue 5, September 2006, at pages 309-315, takes amatrix of users and the films they have rated (each element of thematrix being the rating the user has given to that film), reduces thatmatrix to a binary relation (a formal context) between the users and thefilms they have watched, and generates a concept lattice from thatbinary relation. Each concept in the concept lattice includes a set ofusers, and the set of films which those users have rated. The structureof the concept lattice is then used to limit the number of users whichare considered when finding those users most similar to the target user.

However, the size of a concept lattice is exponential in the number ofusers and films in the binary relation from which it is constructed.This would render concept lattices unsuitable for use in recommendationsystems, in the absence of techniques which reduce the size of theconcept lattice (there is of course, always the option of reducing thepool of users from which the concept lattice is generated, but this islikely to result in less accurate predictions as to whether a user willenjoy a content item).

According to a first aspect of the present invention, there is provideda method of generating recommendations of digital content items for atarget user of a digital content item provision system, said methodcomprising:

generating, for each of a plurality of users of the digital content itemprovision system, a user profile from a concept hierarchy representingdigital content items selected by the user;

comparing said user profiles to identify similar users; and

sending to said target user, recommendations of one or more digitalcontent items based on digital content items selected by similar usersto said target user.

By generating user profiles for a plurality of users of a digital itemcontent provision system, each user profile being derived from a conceptlattice built to categorise digital content items selected by the user,comparing said user profiles to identify similar users; and sending to atarget user, recommendations of one or more digital content items basedon digital content items selected by similar users to said target user,a method of generating recommendations for a target user is provided inwhich only as many digital content items as are found in a binaryrelation which captures the tastes of a single user are included in eachconcept lattice used to generate the user profile. As a consequence, thesize of the concept lattices involved are massively reduced incomparison to prior-art approaches, and the computational efficiency ofthe recommendation method exceeds what has been achieved before.

In some embodiments, said user profile generation comprises:

calculating a relevancy measure for each of one or more concepts in saidconcept hierarchy representing digital content items selected by theuser; and

selecting relevant concepts on the basis of said relevancy measures.

This has the benefit of removing extraneous concepts from the userprofile, and thus simplifying the subsequent identification of similarusers.

Methods of using relevancy measures to remove extraneous concepts fromconcept lattices are known. One approach is simply to only includeconcepts which contain greater than a given percentage of the objects inthe overall formal context. This is seen, for example, in ‘ComputingIceberg Concept Lattices with Titanic’ by Gerd Stumme et al published inData and Knowledge Engineering, Elsevier, 2002, 42(2), pp. 189-222.

Alternative indices for concept selection are suggested in ‘Approachesto the Selection of Relevant Concepts in the Case of Noisy Data’, byMikhail Klimushkin et al, pp 255-266 of vol. 5986 of Lecture Notes inComputer Science, Springer, 2010. In particular, an intensionalstability index is described which defines the intensional stabilityindex of a concept (A,B) as the ratio between the number of sub-contextsof the formal context where B is an intent, to the total number ofsub-contexts of the formal context. An algorithm for calculating thatratio is disclosed in ‘Towards Concise Representation for Taxonomies ofEpistemic Communities’, Camille Roth, Sergei Obiedkov, and DerrickKourie 2006 in Proceedings of the 4th international conference onConcept lattices and their applications (CLA'06), Sadok Ben Yahia,Engelbert Mephu Nguifo, and Radim Belohlavek (Eds.). Springer-Verlag,Berlin, Heidelberg, 240-255. Whilst this index can give concepts lowerin the lattice higher stability values than concepts above them, it iscomputationally burdensome.

In some embodiments, said relevancy measure depends upon the number ofdigital content items found in the concept but not in the largest of theone or more concepts which are lower neighbours of the concept in saidconcept lattice.

This provides a relevancy measure which, like the intensional stabilityindex, captures concepts lower in the hierarchy which might neverthelessbe important in describing the nature of digital content items selectedby the user, but which requires less computing resource than is requiredto calculate the intensional stability index of each concept.

In some embodiments, the method further comprises delivering all or partof said content items to the target user in advance of the user making arequest for the content item. By using the one or more recommendationsof digital content items as a prediction of digital content which theuser is likely to consume in the future, and downloading some or all ofthose digital content items to the users set top box, television orother storage facility located in the user's premises, the user can viewthe digital content items without fear of the live delivery of thosedigital content items being disrupted by the advent of a poor orcongested communications path over the access network to the source ofthe digital content item.

In some embodiments, said calculation of said relevancy measure furthercomprises dividing said number by the number of digital content itemsrepresented in the user profile. This is useful in embodiments wherethere is a need to provide a common criterion for the selection ofrelevant concepts from a plurality of concept lattices.

According to a second aspect of the present invention, there is providedrecommender apparatus comprising:

a user profile generator arranged in operation, for each of a pluralityof users of a digital content provision system, to generate a userprofile from a concept lattice representing digital content itemsselected by the user;

a recommendation engine arranged in operation to find users in saidplurality of users of said digital content system whose user profilesare similar to the user profile of the target user, and to select forrecommendation to the target user, digital content items popular withsaid similar users but not yet provided to the target user; anda recommendation communicator arranged in operation to send arecommendation message to said target user recommending one or more ofthe digital content items selected by the target user.

According to a third aspect of the present invention, there is provideda digital content storage device for consumers of digital content, saiddigital content storage device being arranged to receive saidrecommendation message from recommender message from recommenderapparatus according to the second aspect of the present invention, anddownload some or all of said recommended content.

Examples of digital content storage devices include set-top boxes andsmart televisions.

BRIEF DESCRIPTION OF THE DRAWINGS

There now follows, by way of example only, a description of one or moreembodiments of the invention. This description is given with referenceto the accompanying drawings, in which:

FIG. 1 shows a video-on-demand provision system;

FIG. 2 shows a server computer providing a recommendation facility tousers of the video-on-demand provision system;

FIG. 3 shows an example of a recommendations display provided on thetelevision screen of a user;

FIG. 4 shows a binary relation between a set of films watched by a userand a set of genres to which those films are indicated to belong;

FIG. 5 shows the same information organised as a concept hierarchy;

FIG. 6 is a Hasse diagram representing the concept hierarchy of FIG. 5;

FIG. 7 shows a user viewing history updating process;

FIG. 8 shows a user profile generation process;

FIG. 9 shows a concept lattice refinement process used in the userprofile generation process;

FIG. 10 shows a weighted concept lattice representing a user's viewinghabits;

FIG. 11 shows a user profile generated from the weighted concept latticeof FIG. 10;

FIG. 12 shows a recommendation list generation process;

FIGS. 13A and 13B show example user profiles;

FIG. 14 shows values obtained in a user profile comparison process oncomparing the example user profiles of FIGS. 13A and 13B; and

FIG. 15 shows a recommendation list provision process.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

A video-on-demand communication system (FIG. 1) comprises a head end 10connected by a communication provider's network 12 to a multi-serviceaccess node (MSAN) 14 and a fibre access network 16. The fibre accessnetwork 16 in turn provides connection to a network termination point(not shown) in each of the customer premises connected via that fibrenetwork, the network termination point being further connected viainternal cabling and a residential gateway router (not shown) to a settop box 18. Each set top box 18 outputs a video signal to an associatedtelevision 20 and is controlled by a user using a remote control device22. The control is via a user interface presented on the televisionscreen which the user is able to interact with by using navigation andselection buttons on the remote control 22.

The MSAN 14 is itself connected via a copper access network 24 to othercustomer premises having a similar home network arrangement inside.

Also connected the service provider's network 12 is a Broadband RemoteAccess Server 26 which is connected via a router 28 to the Internet 30.A content provision system including an Internet television servercomputer 32 and a recommendation server computer 34 is connected to theInternet and is configured to deliver television content andrecommendations over the Internet 30 and the communication provider'snetwork 12 to customer premises.

The head end 10 similarly includes a video-on-demand server 36 and arecommender server computer 38.

Each of the recommender server computers 34, 38 is provided withrecommendation software from CD-ROM 40. The software could, of course,instead be downloaded from a predetermined Internet address to theserver computer 34, 38.

Each recommender server computer 34, 38 comprises (FIG. 2) a centralprocessing unit 60, a volatile memory 62, a read-only memory (ROM) 64containing a boot loader program, and writable persistent memory—in thiscase in the form of a hard disk 70. The processor 60 is able tocommunicate with each of these memories via a communications bus 66.

Also communicatively coupled to the central processing unit 60 via thecommunications bus 66 is a network interface card 68. The networkinterface card 68 provides a communications interface between therecommender server computer 34, 38 via communication links to thecommunication provider's network 12 or the Internet 30.

The hard disk 70 of the recommender server computer 34, 38 stores anoperating system program 72, a user viewing-history collector 78, a userprofile generator 80, a recommendation engine 82, and a recommendationmessage provider 82.

Also stored on the hard disk 70 are the user viewing histories 84provided by the user viewing-history collector 78, the user profiles 88derived from those user viewing histories by the user profile generator80, and the recommendation lists for output by the recommendation engine82.

FIG. 3 shows a screen presented to the user showing the top filmrecommendations for a particular user. In this example, nine top filmrecommendations are presented. Picture files are provided as part of therecommendation message provided from the recommendation server computer34, 38. In some embodiments, part or all of the recommended films aredownloaded onto the memory of the user's set top box so that, should theuser choose to watch one of the recommended films, then the films can besmoothly presented by the set top box even in the event of the capacityof the access network 24, 16, communication provider network 12 orInternet 30 being less than that required to stream the film directlyfrom the head end 10 or the Internet television server computer 32.

FIG. 4 shows a table which illustrates the binary relation between a setof films a user has watched, and a set of genres to which those filmsbelong. The genre(s) to which each film belongs might be retrieved froma database such as the Internet Movie Database, or from a databasemaintained by the content provider. The data in this table forms a‘context’ of the type known from Formal Concept Analysis.

Formal Concept Analysis provides techniques for taking a context (FIG.4) and constructing a concept lattice (FIG. 5). Suitable software fordoing that is available from www.upriss.org.uk/fca/fca for example. Eachrow in the table seen in FIG. 5 represents a concept which has twoparts, firstly a list of films (the ‘extent’ of the concept), andsecondly a list of attributes shared by each member of that list offilms (the ‘intent’ of the concept). Adding another attribute to theintent of a first concept will lead to a second concept whose membersare some or all of the films found in the first concept. For example,the films found in concept 10 ({B}) are a subset of the films found inconcept 7 {B, E, F}—the set of films categorised as drama, crime, andadventure is a subset of the set of films categorised as drama andadventure.

In this embodiment, the concept hierarchy includes data indicating thesubset/superset relationships between the different concepts—inparticular, data indicating the one or more lower neighbours of eachconcept is included. However, in other embodiments, this informationcould be deduced when required from the extents or intents of theconcepts.

This hierarchical containment relationship between concepts leads toconcepts being presented as a graph where a concept whose objects are asubset of the larger set of objects of another concept is represented asa child of the concept having the larger set of objects which includesthe subset. An example of such a graph (such graphs are referred to asHasse Diagrams), for the data seen in FIGS. 4 and 5, is seen in FIG. 6.In such diagrams, each node is first labelled only with the objectswhich belong to that node, but not to any child node (the effect beingthat the objects included within a concept can be read off the graph byadding together the objects at the node in question and the objects atall the nodes descending from the node in question. Similarly, each nodeis also labelled with the attribute(s) which characterise that node, butnot any of the predecessor nodes (the effect being that the attributeswhich characterise a node can be read off the graph by adding togetherthe attributes at the node in question, and the attributes of all thenodes encountered when ascending from the node in question).

The user viewing history collector (FIG. 7) is triggered by each requestfor content received from a user's set top box 18. The process involvesfinding 202, from a movie database, the genre of the film which the userhas requested, and then adding 204 the film and the genre information tothe user viewing history. The process then ends 206. This adds a row tothe context table (FIG. 4) for the user who made the content requestwhich triggered the process.

The user profile generator (FIG. 8) is triggered when an appropriatetime period has passed or the user has viewed a certain amount ofcontent. The process begins by fetching 210 the user's viewing history(FIG. 2, 86). A random sample of, say, thirty films viewed by the useris then taken 212, and a concept lattice (FIG. 6) is built. The conceptlattice is then processed to generate 216 a user profile (a step whichis discussed in detail below with reference to FIGS. 8 and 9), and theuser profile (FIG. 2, 88) is stored 218. The user profile generator thenends 220.

The processing of the concept lattice to generate a user profile beginsby setting 230 the current concept to the first node in the lattice datastructure (FIG. 5). Thereafter, an outer set of instructions (232 to250) is repeated for each concept in the lattice, each iteration of theouter set of instructions ending with the incrementation 248 of thecurrent node to the next node in the concept lattice data structure(FIG. 5), and a test 250 to find whether the last node in the conceptlattice has been reached. If the test 250 finds that the last node inthe concept lattice has been reached, then no further iterations of theouter set of instructions are carried out. If, on the other hand, thetest 250 finds that the last node of the concept lattice has not beenreached, then another iteration of the outer set of instructions (232 to250) is carried out for the current concept.

Each iteration of the outer set of instructions (232 to 248) begins bysetting 234 a counter variable to zero, and setting 235 a maximum lowerneighbour concept extent size to zero. An inner set of instructions (236to 242) is then carried out for each concept which is a lower neighbourof the current concept. The inner set of instructions begins byincrementing 236 the counter variable to point to the next lowerneighbour of the current concept. This is followed by a test 238 to findwhether the size of the extent of the current lower neighbour concept isgreater than the current value of the maximum lower neighbour conceptextent size. If it is greater, then the current value of the maximumlower neighbour concept size is set 240 to the size of the extent of thecurrent lower neighbour. A test 242 is then carried out to find whetherthe last of the lower neighbours of the current concept has beenconsidered. If not, another iteration of the inner set of instructionsis performed. If the last of the lower neighbours has been considered, ameasure of the significance of the current concept is calculated 244.The measure, here referred to as the ‘collapse index’, calculates thesignificance of a formal concept (A,B) by determining the minimumpercentage of objects which need be removed from the formal context, inorder for the attribute set B to not be the intent of any formal conceptgenerated from the newly reduced formal context. The greater percentageof objects that need be removed, the ‘stronger’ or more significant theformal concept. The collapse index is a measure of the relevance of theconcept in a description of the films selected by the user. It can beregarded as a weight to be applied to the associated concept insubsequent processing of the concept lattice.

In mathematical terms, given the formal context K=(G, M, I), (C_(i),D_(i)) is a lower neighbour of (A, B), and |C₁|≤|C₂|,≤ . . . ≤|C_(n)|,the collapse index of (A, B) is

$\begin{matrix}{{{ci}\left( {A,B} \right)} = \frac{{A} - {C_{n}}}{G}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The numerator of the fraction is thus a measure of how many objectsfound in the extent of the concept (FIG. 5) are not found in the lowerneighbour concept representing the biggest subset of the extent of thecurrent concept. In other words, the numerator is the size of therelative complement of C_(n) in A (those skilled in the art would beable to provide code executable to calculate the value of the numeratorin that way as an alternative to using the method illustrated in FIG.9).

The denominator of the fraction is the number of objects in the contextas a whole (in this particular example, the number of films in theselection of films chosen from the user's viewing history).

Having calculated the collapse index for the current concept, thecollapse index is added 246 to the lattice data structure (FIG. 5) togenerate an entry in a weighted lattice data structure (FIG. 10). Thecurrent concept pointer is then moved 248 to the next concept in theconcept lattice data structure (FIG. 5), and the test 250 performed.

Once the test 250 finds that all the concepts but the lowest concept(the infimum of the concept lattice) have undergone the outer set ofinstructions (234 to 250), any rows in the weighted concept latticestructure for which the collapse index is less than a predeterminedthreshold are removed 252 from the concept lattice structure. In thepresent example, a threshold (2/11) is chosen which leaves around halfof the original rows in the concept lattice (FIG. 5). Only the extentand intent columns of the selected rows are then used to generate a userprofile (FIG. 11). The user profile thus includes a plurality of rows260, each listing a genre or genre combination 262 which is significantfor the user, in association with a list 264 of the selected films whichhave that genre or genre combination.

On occasion (e.g. after a predetermined time interval for each user), arecommendation list generation process (FIG. 12) fetches 280 the user'sprofile, and finds 282 the top-K similar users based on the applicationof pairwise similarity comparisons with the profiles of other users.Once the top-K similar users are found, in a similar manner to knownautomatic collaborative recommendation systems, the user profiles ofthose users are used to determine a user's interest in content the userhas not previously consumed, and thereby generate a list of recommendedfilms for the user.

A comparison of user profiles may be accomplished by assessing howsimilar the formal concepts in the profile of a first user (e.g. User Awhose profile is seen in FIG. 13A) are to the formal concepts in theprofile of a second user (e.g. User B whose profile is seen in FIG.13B).

For each formal concept in the first profile, a similarity value isobtained between that formal concept and all formal concepts in thesecond profile. This formal concept comparison may be accomplished usingthe Jaccard coefficient on the intent of the pair of formal concepts (inother embodiments the extents might be compared instead of or inaddition to a comparison of the intents). The highest similarity valuefound for a formal concept in the second profile is chosen as the bestmatch for the formal concept in the first profile. This process isrepeated for every formal concept in the profile of the first user. Theaverage of the ‘best matches’ for the formal concepts in profile A isconsidered as a measure of the similarity between the first profile andthe second profile.

Consider as an example the profile of User A (FIG. 13A) and the profileof User B (FIG. 13B). The intent of each formal concept in the profileof User A is compared to all formal concepts in the profile of User B.

In FIG. 15, the Jaccard Coefficients for each of the cross-wisecomparisons of the intents of the profile of User A and the profile ofUser B is shown. The maxima in each column can be seen to be 1.0, 0.33,0.66, 1.0, 0.66 and 1. The average of the best matches is taken as thesimilarity between the two profiles—in this particular instance, 0.778.

By comparing a user with all other users of the video-on-demand orInternet television service, the top-K most similar users can beidentified as those users whose profiles have a top-K similarity scorewith the user's profile.

A list of recommended films for the user is then generated 284 by givingthe movies watched by those similar users a weight corresponding to thesimilarity measures for the similarity of those users to the currentuser. The top N films most commonly found in the other user profiles,but not included within the user's viewing history are then stored bythe recommendation server 34, 38 in its persistent storage 70 (FIG. 2,90). The number of films represented will vary between implementations,but a suitable number might be around 10. The recommendation listgenerated in this way replaces any earlier recommendation lists for thisuser.

As seen in FIG. 15, when the recommender server 34, 38 next receives 260a request from the set-top box 18 seeking a recommendation list for itsuser, the recommendation server 34, 38 responds by sending 262 therecommended film list to the set-top box 18.

In some embodiments, the set-top box 18 then requests some or all of thecontent from a content server—for example from the video-on-demandserver 36 or the Internet television server 32.

Possible variations on the above embodiment include (this list is by nomeans exhaustive):

i) the functionality of the set top box could instead be provided by acomputer in the head end 10, or the Internet television contentprovision system or at another location outside the customer premisesbut within the system of FIG. 1;

ii) the functionality of the set top box could instead provided as anintegral part of the television set;

iii) a recommender engine might be present in only one of the head end10 and the Internet television content provision system. Indeed, onlyone of the head end 10 and the Internet television content provisionsystem might be present in other embodiments;

iv) in the above embodiments, the films selected by a user were arrangedinto a concept hierarchy based on one or more genre(s) attributes.However, in other embodiments, different attributes might be used inconstructing the concept lattices. For example, films might becharacterised by the actors who appeared in them, or by their directorsor screenwriters, or by a combination of attributes (e.g. by the keypeople involved in the the film). In other embodiments, transactionalinformation gathered by the set top box (e.g. time viewed, place viewedetc) might instead be used as attributes to characterise differentfilms;v) whilst, in the above embodiment, the collapse index was calculatedfor every concept in the concept lattice, in other embodiments, it mightbe calculated for a subset of the concepts in the concept lattice—forexample a hybrid approach might be adopted in which a first selectionprocess selects only the concepts having an extent size which exceeds athreshold, that first selection process being followed by a selectionprocess like that described in the above embodiment;vi) whilst in the above embodiment, the digital content items werefilms, the techniques described here are applied in other embodiments todifferent types of digital content items—for example, music recordings,or electronic books;vii) in the above embodiment, a user profile was generated for everyuser of the video-on-demand system. In other embodiments, only selectedusers might have user profiles generated—for example only users whoopt-in or subscribe to a recommendation provision service might haveuser profiles generated for them;viii) in the above embodiment, the similarity between user profiles wascalculated using a ‘best match’ approach. In other embodiments, thesimilarity between the concepts in the two user profiles might becalculated using a cosine similarity measure, in some cases with eachinter-concept similarity being weighted by one or both of the collapseindices for the two concepts;ix) the similarity measure used in the above embodiment is asymmetric—itdepends upon whether the greatest element of each row or each column inFIG. 14 is used. In other embodiments, both ‘best match’ similaritymeasures could be found, and the average or sum of the two taken to bethe overall similarity measure between the two concepts.x) whilst in the above embodiment, the collection of the viewinghistory, the generation of a user profile, the generation ofrecommendations, and the transmission of a message communicating thoserecommendations to a user was achieved by the execution of softwareinstructions on a general purpose processor, in other embodiments one ormore of those functions might be provided by special purpose digitalelectronic hardware, in some cases including programmable logic devices,or application-specific integrated circuits;xi) in the above embodiment, the mere fact that a user had seen, heardor read a digital content item was taken as an indication that the userwould recommend the digital content items to others. However, in otherembodiments, users would be able to give digital content items a rating,and only digital content items which the users gave a high rating (e.g.higher than a threshold) would be included in the formal context.

In summary of the above disclosure, a collaborative recommender systemwhich is based on an application of Formal Concept Analysis isdisclosed. A problem with collaborative recommender systems based onFormal Concept Analysis is that the size of the concept latticesgenerated can be very large. Large lattices both take a great deal oftime to generate, and tend to include extraneous concepts. In thecollaborative recommender systems disclosed herein the size of theconcept lattices used are only of a scale required to capture the tastesof an individual user. This reduces the computational resource requiredby the recommender system markedly. Further improvements are obtained insome embodiments by selecting only relevant concepts from the conceptlattice before building a user profile from the selected concepts. Yetfurther improvements are obtained by the introduction of a relevancemeasure which depends upon the difference between the size of the extentof the concept whose relevance is being measured and the size of thelargest lower neighbour of the concept whose relevance is beingmeasured. This relevance measure can be computed more easily than priorrelevance measures.

The invention claimed is:
 1. A method of operating a processor-basedsystem to generate recommendations of digital content items for a targetuser of a digital content item provision system, said method comprisingoperating said processor-based system to: generate, for each of aplurality of users of the digital content item provision system, a userprofile from a concept lattice representing digital content itemsselected by the user, the concept lattice comprising one or more formalconcepts, each formal concept comprising a data structure including oneor more digital content item identifiers and one or more attributesshared by the identified digital content items, the concept latticebeing based on a Formal Concept Analysis; wherein user profilegeneration comprises: a) calculating a relevancy measure for each of theone or more formal concepts in said concept lattice representing digitalcontent items selected by the user; and b) selecting relevant formalconcepts on the basis of said relevancy measures to use in generatingthe user profile; compare said user profiles to identify similar users;and transmit to said target user, recommendations of one or more digitalcontent items based on digital content items selected by similar usersto said target user; wherein the relevancy measure depends upon thenumber of digital content items found in the formal concept but not inthe largest of the one or more formal subconcepts of the formal concept.2. A method according to claim 1 wherein said calculation of saidrelevancy measure further comprises dividing said number by the numberof digital content items represented in the user profile.
 3. A methodaccording to claim 1 further comprising operating said content provisionsystem to pre-emptively deliver some or all of a recommended digitalcontent item to the target user.
 4. A method according to claim 1further comprising operating said processor-based system to: receivedata representing binary relationships between a set of digital contentitems selected by each user and a set of attributes of those digitalcontent items; and build, for each of said plurality of users, saidconcept lattice representing the digital content items selected by theuser.
 5. The method according to claim 1 further comprising operatingsaid processor-based system to: remove formal concepts of relatively lowrelevancy, as determined from the relevancy measure, from said conceptlattice to generate said user profile.
 6. Recommender apparatuscomprising a processor-based system arranged in operation to: generate,for each of a plurality of users of a digital content provision system,a user profile from a concept lattice representing digital content itemsselected by the user, the concept lattice comprising one or more formalconcepts, each formal concept comprising a data structure including oneor more digital content item identifiers and one or more attributesshared by the identified digital content items, wherein the user profilegeneration comprises calculating a relevancy measure for a plurality offormal concepts in said concept lattice, and removing formal concepts ofrelatively low relevancy from said concept lattice to generate said userprofile; find users in said plurality of users of said digital contentsystem whose user profiles are similar to the user profile of the targetuser, and to select for recommendation to the target user, digitalcontent items popular with said similar users but not yet provided tothe target user; and transmit a recommendation message to said targetuser recommending one or more of the digital content items selected forthe target user; wherein said relevancy measure depends upon the numberof digital content items found in the formal concept but not in thelargest of the one or more formal subconcepts of the formal concept. 7.Recommender apparatus according to claim 6 wherein calculating saidrelevancy measure comprises dividing said number by the number ofdigital content items represented in the user profile.
 8. Therecommender apparatus according to claim 6, wherein the processor-basedsystem is further arranged in operation to: receive data representingbinary relationships between a set of digital content items selected byeach user and a set of attributes of those digital content items; andbuild, for each of said plurality of users, said concept latticerepresenting the digital content items selected by the user.
 9. Adigital content storage device for consumers of digital content, saiddigital content storage device being at least arranged to; receive saidrecommendation message from recommender apparatus comprising aprocessor-based system arranged in operation to: generate, for each of aplurity of users of a digital content provision system, a user profilefrom a concept lattice representing digital content items selected bythe user, the concept lattice comprising one or more formal concepts,each formal concept comprising a data structure including one or moredigital content item identifiers and one or more attributes shared bythe identified digital content items, where in the user profilegeneration comprises calculating a relevancy measure for a plurality lowrelevancy from said concept lattice to generate said user profile; findusers in said plurity of users of said digital content system whose userprofiles are similar to the user profile of the target user, and toselect for recommendation to the target user, digital content itemspopular with said similar users but not yet provided to the target user;and transmit a recommendation message to said target user recommendingone or more of the digital content items selected for the target user;wherein said relevancy measure depends upon the number of digitalcontent items found in the formal concept but not in the largest of theone or more formal subconcepts of the formal concept, and download someor all of said recommended content.
 10. A non-transitory computerprogram element comprising computer program code to, when loaded into acomputer system and executed thereon, cause the computer system toperform the steps of a method of operating the computer system togenerate recommendations of digital content items for a target user of adigital content item provision system, said method comprising operatingthe computer system to: generate, for each of a plurity of users of thedifital content item provision system, a user profile from a conceptlattice representing digital content items selected by the user, theconcept lattice comprising one or more formal concepts, each formalconcept comprising a data structure including one or more digitalcontent item identifiers and one or more attributes shared by theidentified digital content items, the concept lattice being based on aFormal Concept Analysis; wherein user profile generation comprises: (a)calculating a relevency measure for each of the one or more formalconcepts in said concept lattice representing digital content itemsselected by the user; and (b) selecting relevant formal concepts on thebasis of said relevancy measures to use in generating the user profile;compare said user profile to identify similar users; and transmit tosaid target user, recommendations of one or more digital content itemsbased on digital content items selected by similar user to said targetuser; wherein the relevancy measure depends upon the number of digitalcontent items found in the formal concept but not in the largest of theone or more formal subconcepts of the formal concept.
 11. Thenon-transitory computer program element according to claim 10 whereinmethod further comprises operating the computer system to remove formalconcepts of relatively low relevancy, as determined from the relevancymeasure, from said concept lattice to generate said user profile.